Integrated circuits may be advantageously utilized in modern automotive electrical systems, for example, in ignition systems or in seat belt interlock systems, affording substantial cost savings. However, the automotive environment has been found to be an exceptionally harsh one for semiconductor circuits in general, and for integrated circuits in particular. As a result, unexpected problems and requirements have arisen in the design of integrated circuits which must perform reliably in automotive electrical systems and in other high-noise environments. A wide range of temperatures may occur in the automotive environment. Further, a wide range of spurious signals typically occur throughout the wiring of an automotive electrical system. For example, relatively low energy signals of either positive or negative polarity having magnitudes of several hundred volts, hereinafter referred to as "noise" signals, typically occur on wiring lines connecting various sensors to input terminals of integrated circuit devices. Such noise signals may cause malfunctions in the operation of prior art integrated circuit devices, or may even cause destruction of them, and further may destroy discrete semiconductor devices such as power transistors controlled by the integrated circuit. Further, discontinuities in the main power lines of an automotive electrical system, such as interruptions in the connection to the 12 volt automobile battery, may cause severe, high-energy transient voltages, hereinafter called "load dump" voltages, of over 100 volts to occur on the main power lines. The load dump transient voltages may destroy the integrated circuit devices of the prior art in the absence of expensive external protective measures.
Prior art overvoltage protection systems for integrated circuits have typically involved used of an external zener diode connected between the power supply terminals and connected in series with a resistor to the voltage source. During an excessive positive overvoltage, the zener diode breaks over, and the resistor limits the current therethrough. However, especially in automotive applications, the value of the external resistor is necessarily low to prevent a large voltage drop which would impair performance of the integrated circuit. As a result, very large currents flow through the zener diode under breakover conditions, and high cost zener diodes having high power dissipation capability are required. The present invention solves these problems by providing circuitry within the integrated circuit chip which increases the integrated circuit's capability for withstanding high voltages from BV.sub.CEO volts to BV.sub.CES volts.